1. Technical Field
The present invention relates to a conveying system which conveys a workpiece by a robot.
2. Description of the Related Art
A known robot system which includes an external force monitoring function for monitoring a force or torque (hereinafter referred to as “external force”) which is applied to a robot from the outside in order to prevent the robot from being in contact with an object or operator around the robot to cause a serious accident. In such a robot system, safety is ensured by stopping a robot when a detected external force exceeds a predetermined threshold. It is important to take such a safety measure, in particular, in the case of a human-cooperative robot in which a robot and an operator share a workspace.
An external force applied to a robot is detected by a force sensor provided in a base unit of the robot, for example. Specifically, an external force is calculated by subtracting forces (gravity and inertial force) caused by a robot itself when no external force is applied, from a detection value of the force sensor.
In the case of a robot which is used to hold and convey a workpiece to a predetermined position, the detection value of the force sensor is changed by the influence of the workpiece when the robot is holding the workpiece. Therefore, the influence of the workpiece needs to be taken into account to monitor the external force. Specifically, by setting the mass, the center of mass, and the inertia matrix of the workpiece as parameters of the workpiece, forces (gravity and inertial force) applied to the robot caused by the workpiece are calculated. Forces caused by the workpiece are then subtracted from the detection value of the force sensor to calculate an external force.
FIGS. 7A to 7C are drawings illustrating the manner in which a robot 100 according to a related art holds and conveys a workpiece 110. The robot 100 includes a hand 102 at a tip end of an arm, and can hold a workpiece 110 by the hand 102. The robot 100 includes in a base unit a force sensor 104 which detects a force applied to the robot 100.
FIG. 7A illustrates the robot 100 which is positioned at a position where the hand 102 can hold the workpiece 110. The hand 102 is in an open position in which the hand 102 is distant from the workpiece 110 in a horizontal direction. The workpiece 110 is thus supported only by a workpiece support 120.
FIG. 7B illustrates the robot 100 whose hand 102 is moved in the horizontal direction to hold the workpiece 110. The workpiece 110 is still on the workpiece support 120 in the same way as the state in FIG. 7A. The workpiece 110 is thus still supported only by the workpiece support 120.
FIG. 7C illustrates the robot 100 whose position and posture are changed so that the hand 102 moves upward in a vertical direction. In this case, the workpiece 110 is not in contact with the workpiece support 120. The workpiece 110 is therefore supported only by the hand 102, and thus only by the robot 100.
In the following example described with reference to FIGS. 7A to 7C, the mass of the robot 100 is 200 kg, and the mass of the workpiece 110 is 20 kg. For the sake of simplicity, it is assumed that the robot 100 is static. In other words, an inertial force when the robot 100 or workpiece 110 moves is ignored.
In the state of FIG. 7A, the robot 100 is not in contact with the workpiece 110, and a detection value of the force sensor 104 is not influenced by the workpiece 110. Therefore, when no external force is applied, the detection value of the force sensor 104 is equal to the mass of the robot 100, which is 200 kg. In this case, the external force is calculated by subtracting the mass of the robot 100 (=200 kg) from the detection value of the force sensor 104 (=200 kg) to be 0 kg. Since no external force is applied in reality, the calculated value of the external force (0 kg) is correct.
Although, in the state of FIG. 7B, the hand 102 applies forces in the horizontal direction to hold the workpiece 110, no force is applied in the vertical direction. Therefore, when no external force is applied, the detection value of the force sensor 104 is equal to the mass of the robot 100, which is 200 kg.
On the other hand, since the robot 100 holds the workpiece 110, the external force is calculated in consideration of an influence of the workpiece 110 in accordance with the workpiece parameters. In other words the external force is calculated by subtracting the mass of the robot 100 (200 kg) and the mass of the workpiece 110 (20 kg) from the detection value of the force sensor 104 (200 kg) to be −20 kg. In this case, since no external force is actually applied, the calculated value of the external force (−20 kg) is incorrect. Therefore, in such cases, the external force applied to the robot 100 cannot be correctly detected.
In the state of FIG. 7C, the detection value of the force sensor 104 is 220 kg. The external force is obtained by subtracting the mass of the robot 100 (200 kg) and the mass of the workpiece 110 (20 kg) from the detection value (220 kg) of the force sensor 104 to be 0 kg. Therefore, the external force applied to the robot 100 in this case can be correctly detected.
As mentioned above, in a conventional method, an external force cannot be correctly detected during the process from the time when the workpiece is started to be held (see FIG. 7B) to the time when the workpiece is supported only by the robot (see FIG. 7C), and thus the external force monitoring function is not achieved as intended.
WO 2012/077335 A1 discloses a controller of a robot which maintains a stationary state of the robot when the robot holds an object, or releases the object. According to this related art, the robot can be prevented from moving unexpectedly when parameters are switched in accordance with a change of states of an object being held.
However, in the related art disclosed in WO 2012/077335 A1, safety is not taken into consideration during the process from when a workpiece is held and until the workpiece is supported only by the robot.
Therefore, there is a need for a conveying system in which contact accidents are prevented to secure safety during a process from when a robot holds a workpiece and until the robot supports the workpiece.